The invention relates to a macroporous adsorbent, to a process for manufacturing such an adsorbent and to its use for implementing gas separation or purification processes using adsorption, especially PSA, TSA or VSA processes.
Cyclic adsorption processes of the PSA (Pressure Swing Adsorption), TSA (Temperature Swing Adsorption) and VSA (Vacuum Swing Adsorption) type are widely used at the present time for the production of gases. Thus, mention may be made, for example, of the separation of gases from air by preferential adsorption of nitrogen so as to produce oxygen using the PSA or VSA process.
To do this, zeolites, especially of the LTA or FAU type, essentially X or LSX zeolites, are the adsorbents most commonly used in adsorption processes.
Conventionally, the preferred adsorbents for the separation of gases, and more particularly of air, are highly exchanged zeolites, generally exchanged to more than 80%, with alkali or alkaline-earth metals, such as lithium, calcium, strontium and barium, or transition metals, such as aluminium, copper or zinc, or mixtures of these cations.
By way of example, mention may be made of documents U.S. Pat. No. 5,268,023, U.S. Pat. No. 5,174,979, U.S. Pat. No. 4,859,217, U.S. Pat. No. 5,152,813, U.S. Pat. No. 4,481,018, U.S. Pat. No. 5,419,891, EP-A-589406 and EP-A-598391.
Such zeolites are usually synthesized in the form of a powder of micron-sized crystals. However, this pulverulent form cannot be used directly in cyclic gas separation processes or in catalytic processes, since the powder would be entrained by the pressurized gas streams.
Consequently, a forming technique, for example bead formation or extrusion, is required.
For these granulates, the size and form most suited to the process in which they have to be employed are chosen.
In particular, this forming gives the adsorbent its mechanical properties, such as compression strength, and its kinetic properties.
At the present time, no general behavioural law can be established, knowing that it is very difficult to link the parameters of the process to the parameters of the adsorbent. Nevertheless, it is known that the properties of the adsorbent, and especially those linked to its forming, play a paramount role in the overall effectiveness of the processes.
The adsorbents or granulates for separating gases generally consist of a mixture of a zeolitic active phase and an inorganic binder, such as a clay of the bentonite, attapulgite or kaolin type, ensuring mechanical cohesion of the granulates.
One example of manufacture generally used consists in preparing an intimate mixture of a zeolite and a pulverulent binder, to which mixture are added water and, if necessary, organic agents such as, for example, pore formers, fluidizers, plasticizers or any agent that modifies the rheology of the fluid, particularly its viscosity.
The mixture is subsequently formed into rods or strands of various shapes by passing it through a die or an extruder, or formed into beads using a suitable device.
Next, the granulates thus formed are fired at temperatures between 400 and 900xc2x0 C., in fixed-bed or fluidized-bed furnaces, in a gas stream or under vacuum. A judicious choice of the temperature at every moment in this firing step should allow the integrity of the active zeolitic phase to be preserved.
Other processes for manufacturing granulates from solutions containing at least one dry extract exist.
Mention may be made, for example, of document DE-A-4,118,752 which relates to a process for manufacturing granulates, especially AL2O3, ZrO2 and other ceramics, from a mother liquor. In this process, the mother liquor is sprayed and frozen in a cryogenic fluid so as to obtain granulates of 0.04 to 0.4 mm which are then freeze-dried.
Mention may also be made for example of documents FR-A-2,290,246, U.S. Pat. No. 3,862,302, FR-A-1,464,730, U.S. Pat. No. 3,776,988, CH-A-544,083, DE-A-4,420,936 and GB-A-2,092,880 which adopt this type of process.
Now, the productivity of gas separation units using PSA, VSA or TSA cyclic processes is a paramount factor directly related to the production cost of the gases.
One way of increasing this productivity consists in reducing the cycle time so as to produce more gas over a given period.
To do this, it is absolutely essential to use an adsorbent having sufficiently rapid adsorption kinetics, as described in document EP-A-785,020.
One known means of accelerating the mass-transfer kinetics within an adsorbent consists in reducing the hydraulic radius.
However, this approach has several drawbacks, especially that of resulting, if the gas stream is maintained, in a reduction in the permeability, resulting in an increase in head losses within the beds of adsorbents.
The problem which arises is therefore that of developing a novel adsorbent which has faster adsorption kinetics than those of known adsorbents but which results in little or no increase in head losses within adsorbent beds containing such an adsorbent, so as to improve the productivity of units for separating or purifying gases, particularly air.
Furthermore, another object of the invention is also to propose, on the one hand, a method of preparing such an adsorbent and, on the other hand, adsorption processes capable of employing such an adsorbent.
It therefore follows that the present invention relates to the production of adsorbents with a high macroporosity and to their use for the separation of gases by adsorption, and more particularly gases from air.
The term  less than  less than macroporosity greater than  greater than  should be understood to mean, in accordance with the IUPAC data, a pore having a diameter greater than 100 xc3x85.
More specifically, the present invention relates to a macroporous adsorbent comprising pores having a pore volume (Vm) and a mean pore diameter (dm) such that: Vmxe2x89xa71 cc.gxe2x88x921 and dmxe2x89xa71 xcexcm, Vm and dm being determined by mercury porosimetry.
This is because it has been discovered that, surprisingly and unpredictably, such macroporous adsorbents have very rapid adsorption kinetics.
Consequently, the use of these adsorbents in processes for separating gases, especially the gases in air, make it possible to obtain production cycles of very short duration and, consequently, high productivities.
Depending on the case, the macroporous adsorbent of the invention may comprise one or more of the following characteristics:
dm is between 1 xcexcm and 10 xcexcm, preferably dm is between 1 xcexcm and 8 xcexcm and even more preferably dm  greater than 1.6 xcexcm.
Vm is between 1 cc.gxe2x88x921 and 10 cc.gxe2x88x921, preferably Vm is between 1 cc.gxe2x88x921 and 5 cc.gxe2x88x921;
dm is preferably between 1.7 xcexcm and 10 xcexcm, preferably dm is between 1.9 xcexcm and 9 xcexcm;
Vm is preferably between 1.1 cc.gxe2x88x921 and 10 cc.gxe2x88x921 and preferably Vm is between 1.2 cc.gxe2x88x921 and 10 cc.gxe2x88x921;
it comprises an adsorbent active phase and, if necessary, at least one inert binder;
the adsorbent active phase is of the zeolitic type;
the inert binder is chosen from clays, silicas, aluminas, silicates, aluminosilicates and other binders;
the ratio of the proportion by weight of adsorbent active phase to the proportion by weight of inert binder is 50/50 to 95/5, preferably from 70/30 to 90/10;
the adsorbent active phase contains one or more metal cations, preferably alkali metal, alkaline-earth metal or transition metal cations;
the adsorbent zeolite phase is exchanged to at least 80% with one or more metal cations, preferably to at least 90%;
the metal cation is chosen from lithium, calcium, zinc, iron, or any other cation of Groups 1, 2, 7, 10, 11, 12 and f of the Periodic Table of Elements as adopted by IUPAC and preferably the cation is lithium.
Moreover, the invention also relates to a process for manufacturing a macroporous adsorbent according to the invention, in which the adsorbent is manufactured by a spheronization process comprising the steps of:
(a) preparing a mother liquor by mixing at least one dry extract with at least one diluent, the said dry extract containing at least one adsorbent material;
(b) fractionating at least some of the said mother liquor into particles;
(c) freezing at least some of the particles fractionated in step (b) by bringing them into contact with at least one cryogenic fluid at a temperature below xe2x88x92100xc2x0 C.;
(d) removing at least some of the diluent contained in the frozen particles coming from step (c); and
(e) firing at least some of the particles coming from step (d) at a temperature between 300xc2x0 C. and 900xc2x0 C.
Depending on the case, the manufacturing process may comprise one or more of the following characteristics:
the dry extract contains at least one zeolitic-type adsorbent and at least one binder;
the diluent is water;
in step (b), the fractionation is obtained by passing the mother liquor through one or more orifices of controlled size, preferably so as to obtain calibrated cylinders or beads;
in step (c), the cryogenic fluid is liquid nitrogen;
in step (d), the diluent removal is carried out by freeze drying;
in step (e), the firing is carried out at a temperature between 550xc2x0 C. and 750xc2x0 C.;
prior to step (b), a fluidizer or plasticizer is added to the mother liquor so as to modify the rheology, in particular the viscosity, of the said mother liquor and to facilitate its fractionation during step (b).
The macroporous adsorbents according to the invention are particularly well suited to use for the purpose of adsorbing at least one gaseous compound contained in a gas mixture.
In addition, the invention also relates to a process for separating or purifying a gas mixture employing at least one step of adsorbing at least one compound present in the said gas mixture, the adsorption of the said at least one compound being carried out by means of an adsorbent according to the invention.
Depending on the case, the separation or purification process according to the invention may comprise one or more of the following characteristics:
the gas mixture to be purified or separated is chosen from:
air,
mixtures containing oxygen and nitrogen,
mixtures containing carbon monoxide and hydrogen,
hydrocarbons, especially olefins;
it is of the PSA or TSA, preferably VSA, type and the gas mixture is air and the gaseous compound to be separated is oxygen.